DESCRIPTION: Dendritic cells (DCs) are potent antigen presenting cells that play a crucial role in directing adaptive immune responses. Because the Gl tract is exposed to both benign antigens from food and commensal bacteria and to sinister antigens from pathogens, intestinal DCs play a particularly important role in guiding immune responses. Recently a population of dendritic cells located directly underneath the intestinal epithelia has been described. These epithelial associated DCs (EA-DCs) extend dendrites between epithelial cells to sample the luminal contents and therefore have initial and crucial encounters with luminal antigens. The focus of this proposal is to understand how these EA-DCs localize to the intestine, sample antigen, and direct immune responses to protect from pathogenic infections while at the same time avoiding the untoward effects of uncontrolled inflammatory responses. Prior studies have demonstrated that the sampling behavior of the EA-DCs has regional variation in the intestine. Our preliminary studies demonstrate that the EA-DCs are comprised of two subtypes, and that inflammatory EA-DCs are more prominent in the distal small intestine, and tolerogenic EA-DCs are more prominent in the proximal small intestine. We hypothesize that the phenotype of each EA-DC subtype is intrinsic to that subtype regardless of localization within the intestine and that luminal microbial stimuli direct the localization and sampling behavior of the individual EA- DC subtypes. This hypothesis will be investigated by performing the following specific aims: (1) Compare the phenotype and function within EA-DCs subtypes residing in the proximal to those residing in the distal small intestine. These studies will use flow cytometry, real time PCR, Western blots, and in vitro culture assays on flow cytometric sorted EA-DC subtypes from the proximal and distal intestine. (2) Evaluate the role of stimuli derived from luminal microbiota in directing the regional localization and sampling behavior of individual EA-DC subtypes. These studies will evaluate the EA-DC populations in mice with various degrees of luminal microbial stimuli using intravital microscopy, flow cytometry, real time PCR, and Western blots on flow cytometric sorted EA-DC populations. PUBLIC HEALTH RELEVANCE: Intestinal infections are the number one cause of death in the world but even in a healthy person, the intestine is the home of a large population of beneficial bacteria. We hope to shed light on how the intestinal immune system decides when to attack the harmful bacteria and when to tolerate the beneficial bacteria as a clue to understand how to treat chronic inflammatory diseases where the immune system cannot seem to tell the difference between friend or foe.